1. Field of the Invention
The present invention relates to a three-dimensional visual sensor or, in particular, to a three-dimensional visual sensor for radiating a spot of light or a slit of light on the surface of an object and for measuring the three-dimensional position of specific points on the object. The three-dimensional visual sensor according to the invention is used for various applications and in combination with a robot, for example.
2. Description of the Related Art
A conventional method of determining the three-dimensional position of an object to be measured is known. In the method several end points of the object are determined by use of a spot of light (hereinafter referred to also as “a pseudo-slit of light”) or a slit of light scanned in planar form, as described in (i) and (ii) below.
(i) Two or more slit of light beams are projected into a hole formed in a given flat metal plate, and at least three end points are detected thereby to determine the three-dimensional position and the direction of the surfaces of the hole. This method is disclosed, for example, in Japanese Unexamined Patent Publication No. 6-258024.
(ii) Template matching is conducted using a scanned spot of light (pseudo-slit of light) or a slit of light with the shape of the lights on an image as a template. In this way, the scanned spot of light or the slit of light is specified, and the three-dimensional position of the detected points included therein are determined. This method is disclosed, for example, in Japanese Unexamined Patent Publication No. 10-105717.
Various methods of calibration are well known for a three-dimensional visual sensor used to measure the three-dimensional position of a specified point of an object by radiating the spot of light or the slit of light on the surface of the object. One of the methods is disclosed, for example, in Japanese Unexamined Patent Publication No. 10-063317.
In the method (i) described above, the higher the measurement accuracy, the higher the accuracy at which the portion where the hole crosses the slit of light or the pseudo-slit of light formed by the scanning spot of light is detected. The portion where the hole and the slit of light cross each other, however, is actually where it is most difficult to specify the slit of light. Although some improvement is expected by elaborate design efforts, therefore, it is difficult to accurately specify the slit of light. Also, depending on the surface condition or the color of the object, the intersection point is often blurred and invisible or develops a halation to an indistinguishable degree.
In the method (ii), on the other hand, template matching is conducted on an image. Depending on the color or the surface condition of the object, however, the slit line is thickened or blurred. Even in the case where the center of the slit of light is accurately defined, an image of a different shape from the template model is often formed, with the result that steady detection is difficult. Further, the portion near the end of the image is easily affected by lens distortion. The employment of a lens having a long focal length to avoid this inconvenience would lead to the problem that the visual field is so narrow that even a slight displacement of the object makes the slit of light invisible.
Accordingly, the object of this invention is to solve the problem of the related art described above. Specifically, this invention is intended to improve the three-dimensional visual sensor for measuring the three-dimensional position of a specific point of an object by radiating the spot of light or the slit of light on the surface of the object. Even in a case where accurate measurement of a three-dimensional position is difficult in the related art, this invention makes it possible to determine the three-dimensional position of a feature point on a reference light plane.